1. Field of the Invention
The present invention is in the technical field of energy storage devices. More particularly, the present invention is in the technical field of rechargeable batteries employing an iron electrode.
2. State of the Art
Iron electrodes have been used in energy storage batteries and other devices for over one hundred years. In particular, iron electrodes are often combined with a nickel-based positive electrode in alkaline electrolyte to form a nickel-iron (Ni—Fe) battery. The Ni—Fe battery is a rechargeable battery having a nickel(III)oxy-hydroxide positive electrode in combination with an iron negative electrode with an alkaline electrolyte such as potassium hydroxide.
The Ni—Fe battery is a very robust battery which is very tolerant of abuse such as overcharge and overdischarge and can have a very long life. It is often used in backup situations where it can be continuously trickle-charged and last more than 20 years.
Traditionally, the iron electrode active material is produced by dissolving pure iron powder in sulfuric acid, followed by drying and roasting to produce iron oxide (Fe2O3). The material is washed and partially reduced in hydrogen and partially oxidized to give a mix of Fe and magnetite (Fe3O4). Additives such as FeS may be added to the active material mass. The negative electrode structure is typically that of a pocket plate construction wherein the active material is introduced into the current collector. The current collector is made up of steel strips or ribbons that are perforated and nickel plated and the strip formed into a tube or pocket with one end left open for introduction of the active material (D. Linden and T. Reddy, Editors, “Handbook of Batteries, Third Edition”, McGraw-Hill, © 2002). Alternatively, fine iron powder can be sintered under a reducing atmosphere to yield a sturdy electrode shape.
Both of these methods for producing iron electrodes are expensive, lead to low active material utilization, and poor specific energy. As a result, Ni—Fe batteries have largely been displaced by other battery technologies due to the high cost of manufacturing and low specific energy. While the technology of preparing iron electrodes is well known and the current preferred process for making these electrodes is a pocket design, pocket design electrodes are not cost effective and are complex in manufacturing. Although the theoretical capacity of an iron electrode is high, in practice only a small percentage of this is achieved due to the poor conductivity of iron oxide. In a pocket electrode design, loss of contact to the external matrix surface results in increased polarization and a drop in cell voltage. To avoid this, large amounts of conductive material such as graphite must be added to the active material, further increasing cost and lowering energy density. The industry would be well served by a low cost, high quality and high performance iron electrode design.
The substrate in an electrode is used as a current conducting and collecting material that houses the active material (iron) of the electrode in a mechanically stable design. In current pocket electrode designs, the substrate encompasses the active material and holds the material between two layers of conductor, therefore requiring two substrates per electrode. In this process, pockets are formed by interlocking two perforated Ni-coated strips into which the active material is compressed. While such a design offers long life, the energy density is poor.
An alternative process utilizes a porous sintered structure of iron powder, which is filled with iron hydroxide by either an electrochemical process or by impregnation of the pores with an appropriate iron salt, followed by immersion in alkaline solution. Such electrodes suffer from poor active material loading and corrosion of the iron porous plaque during impregnation, leading to limited life.
To address these short-comings, U.S. Pat. No. 4,236,927 describes a process whereby iron powder and a reducible iron compound are mixed together and sintered into a stable body. This mixture is then sintered at high temperature to form a plate of desired shape. While this eliminates the need for a sintered plaque substrate or pockets of Ni-coated steel, it requires high temperature sintering under hydrogen atmosphere. Such processes add considerable complexity and cost in volume manufacturing.
Other forms of electrode production are known in the art, particularly electrodes of a pasted construction. This type of electrode typically incorporates a binder with the active material, which can then be coated onto a two or three dimensional current collector, dried, and compacted to form the finished electrode.
U.S. Pat. No. 3,853,624 describes a Ni—Fe battery incorporating iron electrodes employing a metal fiber structure which is loaded with sulfurized magnetic iron oxide by a wet pasting method. The plates are electrochemically formed outside the cell to electrochemically attach the iron active material to the plaque structure. Such a process in unwieldy in high volume manufacturing and adds to product cost.
U.S. Pat. No. 4,021,911 describes an iron electrode wherein the iron active mass is spread onto a grid and rolled and dried. The electrode is then treated with an epoxide resin solution to form a solid reinforcing film-like layer on the electrode surface. However, it can be expected that such a surface film would contribute to an insulating nature to the electrode surface, significantly increasing charge transfer resistance and lowering the cell's ability to sustain high charge and/or discharge rates.
Similarly, PTFE has been proposed as a binder system for paste type electrodes for alkaline batteries. U.S. Pat. No. 3,630,781 describes the use of a PTFE aqueous suspension as a binder system for rechargeable battery electrodes. However, to maintain the PTFE powder in suspension, it is necessary to add surfactants to the suspension, which must be removed from the resultant electrode by extensive washing, adding cost and complexity to the manufacturing process. An alternative approach for a PTFE-bonded electrode is described in U.S. Pat. No. 4,216,045 using fluorocarbon resin powder to form a sheet which can be attached to a conductive body. However, the use of PTFE results in a water-repellent surface, which while beneficial in a recombinant battery such as NiCd or NiMH, is detrimental to the performance of a flooded Fe—Ni battery where good contact between the electrode and electrolyte is beneficial.
Pasted electrodes using various binders have been proposed for alkaline electrodes, most particularly for electrodes employing hydrogen-absorbing alloys for NiMH batteries (for example U.S. Pat. No. 5,780,184). However, the desired properties for these electrodes differ significantly from those desired for a high capacity iron electrode. In the case of the MH electrode, high electrode density (low porosity) is required to maintain good electrical contact between the alloy particles and to facilitate solid-state hydrogen diffusion in the alloy. By contrast, high porosity is desirable for iron electrodes due to the low solubility of the iron oxide species. Hence, binder systems developed for other types of alkaline electrodes have not been optimized for Fe—Ni batteries and hence have not found commercial application.
Polyvinyl alcohol (PVA) is a water-soluble synthetic polymer prepared by partial or complete hydrolysis of polyvinyl acetate to remove acetate groups. Due to its excellent resistance to alkaline environments, PVA has been proposed for use in separators for alkaline batteries (e.g. U.S. Pat. No. 6,033,806). Additionally, PVA has been employed as a binder material for certain alkaline battery electrodes, most notably, nickel hydroxide electrodes. However, these electrodes are characterized by a three dimensional structure such as a foam or felt substrate that provides mechanical stability to the finished electrode. Therefore, it is not critical to form a fibrous polymer network to stabilize the active material within the electrode structure.
PVA has generally not been found to be an effective binder in electrode structures that rely on a single substrate material such as nickel plated strip (NPS), expanded metal, or wire mesh. This is because of the relatively poor binding properties relative to more fibrous polymers such as PTFE. PVA does not provide sufficient binding force to prevent premature shedding of active material and delamination from the substrate. For these reasons, more fibrous binders are typically employed, most notably PTFE. However, PTFE suffers from several drawbacks. Since PTFE is not water soluble, it must be introduced into the paste in a colloidal suspension. Such a suspension is unstable and can flocculate, rendering the suspension unusable. A surfactant is used to maintain the PTFE in a colloidal suspension, and such a surfactant can cause foaming during processing and must be completely removed from the electrode prior to cell assembly. Similarly, the suspension can stratify, requiring regular stirring of stored material. A further property of PTFE as a battery electrode binder is that it imparts a hydrophobic nature to the electrode surface. While this may be a desirable property in batteries requiring gas recombination, such as NiCd or NiMH, it is undesirable in a Ni—Fe battery, where such hydrophobicity may hinder access of the electrolyte to iron active material. Other binders have been used in alkaline batteries such as various rubbers, but these materials are generally not water soluble, requiring the use of organic solvents, adding cost and complexity to manufacturing.
PVA has recently been proposed as a component to a binder system for lithium ion batteries employing anode materials that are subject to large volume changes, but requires the addition of polyurethane to provide semi-interpenetrating polymer network (U.S. Pat. No. 7,960,056).
The object of this present invention is to provide a high quality and low cost iron electrode that overcomes the limitations of current state-of-the-art pocket and/or sintered iron electrodes.